Group III-Nitrides have gained a lot of importance for the last decade, for example in semiconductor processing. Examples of applications are High Electron Mobility Transistors (HEMT) for high power and high frequency applications, blue LEDs, etc.
An example of group III-nitrides are Ge-nitrides. Ge-Nitrides have been first synthesized by Johnson in 1930 (J. Am. Chem. Soc. 52, 5160 (1930)) by exposing a powder sample of Ge to an ammonia atmosphere at high temperatures (˜700° C.). This material was later on identified to have the phenacite hexagonal structure β-Ge3N4 (Z. Anorg. Allg. Chem. 241, 32 (1939)).
In the mean, time, a lot of experimental work has been devoted to the formation of stable GeN layers, e.g. Ge3N4 layers, by different methods. The layers obtained so far on crystalline Ge are either amorphous or polycrystalline. Two different hexagonal structures as well as one rhombic structure have been identified. In Table 1, lattice parameters are listed for different crystalline GeN structures as mentioned in literature (Molina, Int. J. Quantum Chem. 80, 249 (2000)).
TABLE 1Lattice parameters for different crystalline GeN structures.ACBDensityStructure[Å][Å][Å][g/cm3]β-Ge3N48.0328.0785.287α-Ge3N48.2025.9415.254c-Ge3N413.848.189.065.287
A way of forming GeN is by nitridation of a germanium substrate. It is known that nitridation of Ge using a N2 plasma cell in a UHV (ultrahigh vacuum) chamber at temperatures of between 100° C. and 550° C. results in the formation of amorphous Ge3N4 (Wang et al. Applied Physics Letters 89, 022105 (2006)). It was found that after annealing at 600° C. in vacuum, this layer completely evaporated, implying that the Ge3N4 film is not stable at temperatures above 600° C. Nitridation of Ge layers in UHV by a N2 plasma cell is known to lead to amorphous GeN (T. Maeda, T. Yasuda, M. Nishizawa, N. Miyata, Y. Morita, S. Takagi, Applied Physics Letters Volume 85, 15, 2004).
Exposure of Ge to an ammonia (NH3) atmosphere at high temperatures has been used to form GeN. Johnson et at (J. Am. Chem. Soc. 52, 5160 (1930)) have shown that by exposing a powder sample of Ge to an ammonia atmosphere at high temperatures (˜700° C.) GeN can be synthesized. However the GeN obtained is in powder form and does not form an epitaxial layer on top of a crystalline Ge layer. It was furthermore reported in 1967 (V. F. Synorov, E. A. Kuznetsova, and N. M. Aleinikov, Soviet Physics Journal, Vol. 10, No. 3, pp. 7-11, 1967) that annealing of Ge under ammonia atmosphere leads to the formation of many phases: hexagonal GeN, a loose layer of rhombohedral GeN and GeO2. It is therefore known that exposure of Ge at high temperature to an ammonia atmosphere leads to the formation of polycrystalline GeN.